HHH-8406 - fix for EntityManagerFactoryRegistry not being thread safe

hibernate-entitymanager/src/main/java/org/hibernate/jpa/internal/EntityManagerFactoryRegistry.java

@@ -139,10 +139,11 @@ public class EntityManagerFactoryRegistry {
 		if (entityManagerFactorySet == null) {
 			throw new HibernateException( "registry does not contain entity manager factory: " + name);
 		}
-
+		synchronized (entityManagerFactorySet) {
 			if (entityManagerFactorySet.size() > 1) {
 				throw new HibernateException( "registry contains more than one (" + entityManagerFactorySet.size()+ ") entity manager factories: " + name);
 			}
 			return entityManagerFactorySet.iterator().next();
 		}
 	}
+}